(1) Field of the Invention
The present invention relates to methods for forming porous layers on the outer surfaces of electrode hoops of nonaqueous electrolyte secondary batteries or other batteries.
(2) Description of Related Art
As portable electronic devices and cordless electronic devices have come to be more widely used, expectations are growing for lightweight nonaqueous electrolyte secondary batteries with high energy density. However, an active material of such a nonaqueous electrolyte secondary battery is made of highly reactive lithium, and therefore a short circuit between a positive electrode and a negative electrode under abnormal conditions generates heat. Due to the generated heat, a resinoid microporous membrane separator for isolating the positive electrode from the negative electrode melts around the area in which the electrodes are short-circuited, thereby increasing the area in which the electrodes are short-circuited and thus generating abnormal superheat. In order to prevent this, a technique is being used in which, in case of the occurrence of a short circuit between electrodes, superheat due to an increase in the area where the electrodes are short-circuited is suppressed by combining a porous refractory layer having a refractory resin or an inorganic oxide as the main ingredient with the resinoid microporous membrane separator.
Such a porous refractory layer is formed on the outer surface of a positive electrode or a negative electrode (hereinafter, referred to as an “electrode”) without loss of the design capacity of a battery to have a thickness of 2 through 10 μm. A method in which a coating fluid serving as a precursor of a porous refractory layer is transferred to a gravure roll provided with a plurality of grooves and the transferred coating fluid is applied to the outer surface of a target electrode hoop (hereinafter, referred to as “gravure method”) is preferably employed as a method for forming a layer of a thickness as described above with high accuracy. In order to realize the gravure method, the direction in which an electrode hoop travels is allowed to become opposite to the direction of rotation of a gravure roll. In this way, a thin coating membrane can be formed on the outer surface of an object to be coated with a coating fluid with high accuracy.
By the way, from the viewpoint of improving the productivity of electrodes, electrodes each serving as a base of a porous refractory layer are typically processed in the following manner: A core that is several times as wide as each electrode itself is prepared; a plurality of linear portions of a mixture layer containing an active material are formed on the core; and then the core is cut into pieces each having one of the linear portions of the mixture layer. In view of the above, a plurality of porous refractory layers need be formed in consideration of the shape of each electrode.
A method in which a masking tape is bonded to the outer surface of a core, then a mixture layer is formed on the masking tape, and thereafter the masking tape is removed has been disclosed, as a method for forming a mixture layer on the outer surface of a core with high accuracy, in Japanese Unexamined Patent Publications Nos. 2005-183181, 2005-216722 and 2005-216723. Application of this method allows a plurality of linear portions of a porous refractory layer each having a predetermined width to be formed on the outer surface of an electrode.